1. Field of the Invention
The invention relates to a signal synthesizing circuit to prevent copying and a detecting circuit and, more particularly, to a signal synthesizing circuit to prevent copying and a detecting circuit which are suitable for use in a video tape recorder (hereinafter, referred to as a VTR).
2. Description of the Prior Art
An example of the prior art to prevent copying of a video signal is shown in FIG. 1. That is, a video signal reproduced by a VTR 75 on the reproducing or playback side is separated into component signals Y, PB, and PR and supplied from the VTR 75 on the playback side to a VTR 76 on the recording side through terminals 78 to 80. A copy inhibition signal SCG to specify permission or inhibition of copying the video signal is supplied from the VTR 75 on the playback side to the VTR 76 on the recording side through a terminal 77. In the VTR 76 on the recording side, the video signal is recorded or the recording of the video signal is inhibited in accordance with the content of the copy inhibition signal SCG.
According to the above prior art, in place of the copy inhibition signal SCG, when a signal corresponding to a copy permission signal, for instance, (DC voltage at a high level of +5 V) is supplied to a terminal for receiving the copy inhibition signal provided for the VTR 76 on the recording side, there is a problem in that copy of the video signal is enabled irrespective of the content of the copy inhibition signal SCG.
In the above prior art, where circuitry, cables, terminals, and the like of another system different from those for the video signal are provided and the copy inhibition signal SCG must be supplied from the VTR 75 to the VTR 76. Thus, the system becomes complicated and costs rise. Further, the above prior art has a problem in that it is difficult to use as a general standard because the copy inhibition can be extremely easily cancelled.